Check this out:
http://www.rit.edu/~flwstv/galileo.html
"Galileo is universally known by his first name only, his full name being Galileo Galilei. The form of the name arose from a Tuscan habit of using a variation of the last name for the first name of the oldest son."
I'd actually never thought about Galileo's name much before. But thanks to your posting, I just had to find out the answer. :)
-- Nanaea